Fig. 1: Excitonic condensation in broken-gap 2D bilayer heterostructures.
From: Heterobilayers of 2D materials as a platform for excitonic superfluidity
a Band alignment in a semi-metallic system with distinct electron and hole pockets necessary for exciton condensation. b For such a system with a negative gap (Eg) or when Eg < Eb (the exciton binding energy), the semi-metallic state becomes unstable and opens a gap for an arbitrarily weak electron-hole attraction. The semimetal now transitions to an excitonic insulator, with a gap of 2Δ in the excitation spectrum. c For a broken-gap band alignment, the electron is transferred to the conduction band of the acceptor layer B, with a hole remaining in the valence band on the donor layer A. In a staggered-gap alignment, band overlap can be tuned by external perturbation.
